Among fluorides, a calcium fluoride (CaF2) single crystal body, a magnesium fluoride (MgF2) single crystal body and the like have been used in the optical field limited to, for example, the vacuum ultraviolet region of wavelengths of 160 nm or less, or the extreme infrared region of wavelengths of 3 μm or more. Fluorides have been used as optical materials such as a window material, a lens and a prism for such specific wavelength regions wherein light does not pass through high-purity quart glasses and optical glasses widely used in the market. Therefore, the amount of demand thereof is naturally small.
Among them, a magnesium fluoride (MgF2) noticeably shows a birefringence phenomenon, and therefore, it is not suitable for a lens or a prism. It is rarely used as a window material for transmitted light, and the demand thereof is very small.
In order to manufacture a MgF2 single crystal body for optical use which is substantially only one usage thereof, the Bridgman-Stockbarger method (crucible descending method) being a kind of so-called zone melting, or the Czochralski method (single crystal pulling method) is adopted.
In the Bridgman-Stockbarger method, wherein a vertical heating furnace is used, a crucible is charged with a raw material powder. The crucible is allowed to slowly descend from the upper side in the heating furnace and pass through a soaking zone, by which the state of the raw material in the crucible is shifted from ‘heating’ to ‘melting’, and then to ‘solidification (crystallization)’, and then to ‘crystal growth’, gradually from the lower portion thereof toward the upper portion thereof. Meanwhile, vaporized gases generated through sublimation of the raw material or gases existing in voids among raw material particles are released from the top side of the crucible into the furnace so as to reduce residual bubbles in the crystal generated in solidification.
In the Czochralski method, wherein a heating furnace and a crucible are used, a raw material in the crucible is melted. A seed crystal is dipped from the upper side of the crucible in the upper surface portion of the melt, and the melt of the dipped portion is slowly pulled up with rotation and solidified (crystallized) so as to pull a single crystal upward.
Both of these methods are appropriate to reduce residual bubbles in the case of a raw material such as a fluoride which easily sublimate in the heating process. However, the above-described crystal growth takes a long time, for example, it takes several months to produce a single crystal of about 10 kg. Thus, the productivity thereof is extremely low, resulting in very high producing cost.
Generally speaking, there are very few cases wherein a fluoride is used for other than such optical uses. The CaF2 single crystal body, or a single crystal body of lithium fluoride (LiF) and aluminum fluoride (AlF3) has been rarely used as a shield to neutrons, one of radioactive rays. However, such single crystal bodies have plane orientation dependency of moderation performance originated in crystal orientation and ununiformity due to structural defects such as a subgrain or a subboundary. Moreover, in the case of MgF2, it has a large birefringence. Therefore, it is necessary to distinguish each characteristic of these materials for proper use, resulting in difficulties in use.
Since the above fluoride single crystal bodies include a defect portion such as a subgrain or a subboundary having a loose crystal structure and a slightly low density, in many cases, the density thereof is slightly lower than the theoretical density (i.e. true density). For example, in the case of MgF2, the true density thereof is 3.15 g/cm3, while the densities of actual single crystals are 3.130-3.145 g/cm3. Many of them have a relative density of 99.4%-99.8%.
Hereinafter, radioactive rays in relation to the usage of the present invention are briefly described. The radioactive rays are roughly classified into alpha (α)-rays, beta (β)-rays, gamma (γ)-rays, X-rays, and neutrons. The power passing through a substance (penetrability) gradually increases in this order.
The neutrons which have the highest penetrability among them are further classified into the below-described groups, for example, according to the energy level which they have. The energy level each type of neutrons has is shown in parentheses, and the larger the value is, the higher the penetrability is. In the order of the lowest penetrability, they are classified into cold neutrons (up to 0.002 eV), thermal neutrons (up to 0.025 eV), epithermal neutrons (up to 1 eV), slow neutrons (0.03-100 eV), intermediate neutrons (0.1-500 keV) and fast neutrons (500 keV or more).
Here, there are various views concerning the classification of neutrons, and the energy values in the parentheses are not precise. For example, there is a view that mentions 40 KeV or less, which is within the above energy region of intermediate neutrons, as the energy level of epithermal neutrons.
The typical effective utilization of neutrons is an application to the medical care field. In particular, the radiation therapy in which tumor cells such as malignant cancers are irradiated with neutrons so as to be broken has been coming into general use in recent years. However, in order to obtain medical effects in the present radiation therapy, neutrons of a certain high energy level must be used, so that in the irradiation of neutrons, the influence on a healthy part other than an affected part of a patient cannot be avoided, leading to side effects in some cases. Therefore, in the present situation, the application of the radiation therapy is limited to severe patients.
When a normal cell is exposed to high-energy-level neutrons, its DNA is damaged, leading to side effects such as dermatitis, anemia due to radiation and leukopenia. Furthermore, in some cases, a late injury may be caused some time after treatment, and a tumor may be formed and bleed in the rectum or the urinary bladder.
In recent years, in order not to cause such side effects and late injuries, methods of pinpoint irradiation on a tumor have been studied. Examples thereof are: “Intensity Modulated Radiation Therapy (IMRT)” in which a tumor portion is three-dimensionally irradiated accurately with a high radiation dose; “Motion Tracking Radiation Therapy” in which radiation is emitted to motions in the body of a patient such as breathing or heartbeat; and “Particle Beam Radiation Therapy” in which a baryon beam or a proton beam each having a high remedial value is intensively emitted.
As a characteristic of a neutron, the half-life thereof is short, about 15 min. The neutron decays in a short period of time, releases electrons and neutrinos, and turns into protons. And the neutron has no charge, and therefore, it is easily absorbed when it collides with a nucleus. The absorption of neutrons in such a manner is called neutron capture, and one example of an application of neutrons to the medical care field by use of this feature is the below-described “Boron Neutron Capture Therapy (hereinafter, referred to as BNCT)”, a new cancer therapy which is recently gaining attention.
In this BNCT, by causing tumor cells such as malignant cancers to react with a boron drug which is injected into the body by an injection, a reaction product of a boron compound is formed in the tumor portion. The reaction product is then irradiated with neutrons of an energy level which has less influences on a healthy part of the body (desirably comprising mainly epithermal neutrons, and low-energy-level neutrons being lower than epithermal neutrons). And a nuclear reaction with the boron compound is caused only within a very small range, resulting in making only the tumor cells extinct. Originally, cancer cells easily take boron into them in the process of vigorously increasing, and in the BNCT, by use of this feature, only the tumor portion is effectively broken.
This method has been attracting attention as an excellent radiation therapy since quite long before because of small influences on a healthy part of a patient, and has been researched and developed in varied countries. However, there are wide-ranging important problems on the development such as development of a neutron generator and a device for a selection of the types of neutrons to be remedially effective, and avoidance of influences on a healthy part other than an affected part of a patient (that is, formation of a boron compound only in a tumor portion). Therefore, the method has not come into wide use as a general therapy.
Significant factors in terms of apparatus why it has not come into wide use are necessity to develop a neutron generating device for exclusive use, insufficient downsizing of the whole apparatus including the generating device and insufficient enhancement of its performance.
For example, there is a latest system of the BNCT, which a group with Kyoto University as the central figure has been promoting (Non-Patent Document 1 and Non-Patent Document 2). This system comprises an apparatus for medical use only, having a cyclotron accelerator as a neutron generator which is exclusively installed without being attached to an existing nuclear reactor. One report says that the accelerator alone weighs about 60 tons, and its size is large. In the cyclotron system, protons are accelerated by use of a centrifugal force in a circular portion of the cyclotron. Accordingly, in order to efficiently generate neutrons, it is required to make the diameter of the circular portion large so as to obtain a large centrifugal force. That is one of the reasons why the apparatus is large.
Furthermore, in order to safely and effectively utilize the generated radiation (mainly neutrons), a radiation shield such as a shielding plate (hereinafter, referred to as a moderator) is required. As moderators, CaF2 and polyethylene containing LiF, as well as Pb, Fe, Al and polyethylene, are selected. It cannot be said that the moderation performance of these moderators is sufficient, and in order to realize required moderation, the moderator becomes quite thick. Therefore, the moderation system device portion including the moderator is also one of the reasons why the apparatus is large.
In order to allow this BNCT to come into wide use in general hospitals, downsizing of the apparatus is necessary. In addition to further downsizing of the accelerator, to improve the remedial values by developing a moderator having high moderation performance and achieve downsizing of the moderation system device by the improvement of moderation performance is an urgent necessity.
The moderator which is important for downsizing a BNCT apparatus and improving remedial values is briefly described below.
As described above, in order to safely and effectively utilize radiation, it is necessary to arrange a moderator having the right performance in the right place. In order to effectively utilize neutrons having the highest penetrability among radioactive rays, it is necessary to accurately know the moderation performance of every kind of substances to neutrons so as to conduct effective moderation.
One example of the selection of particle beam types in order to effectively utilize neutrons for medical care is shown below. By removing high-energy neutrons which adversely influence the body (such as fast neutrons and a high-energy part of intermediate neutrons) as much as possible, and by further reducing extremely-low-energy neutrons having little medical effect (such as thermal neutrons and cold neutrons), the ratio of neutrons having high medical effects (such as a low-energy part of intermediate neutrons and epithermal neutrons) is increased. As a result, a particle beam effectively utilized for medical treatment can be obtained.
The low-energy part of intermediate neutrons and epithermal neutrons have a relatively high invasive depth to the internal tissues of a patient. Therefore, for example, in the case of irradiating the head with the low-energy part of intermediate neutrons and epithermal neutrons, as far as the tumor is not present in a considerably deep part, without craniotomy required, it is possible to carry out effective irradiation to an affected part in an unopened state of the head.
On the other hand, when the extremely-low-energy neutrons such as thermal neutrons are used in an operation, because of their low invasive depth, craniotomy is required, resulting in a significant burden on the patient.
In order to improve remedial values in the BNCT, it is required to irradiate an affected part with a large quantity of neutrons comprising mainly epithermal neutrons and some thermal neutrons. Specifically, an estimated dose of epithermal neutrons and thermal neutrons required in cases where the irradiation time is in the order of one hour, is about 1×109 [n/cm/sec]. In order to secure the dose, it is said that as the energy of an outgoing beam from an accelerator being a source of neutrons, about 5 MeV-10 MeV is required when beryllium (Be) is used as a target for the formation of neutrons.
The selection of particle beam types through moderators of every kind in a neutron radiation field for BNCT using an accelerator is described below.
A beam emitted from the accelerator collides with a target (Be, in this case), and by nuclear reaction, high-energy neutrons (fast neutrons) are mainly generated. As a method for moderating the fast neutrons, using lead (Pb) and iron (Fe) each having a large inelastic scattering cross section, the neutrons are moderated to some extent. In order to further moderate the neutrons moderated to some extent (approximately, up to 1 MeV), optimization of the moderator according to the neutron energy required in the radiation field is conducted.
As a moderator, aluminum oxide (Al2O3), aluminum fluoride (AlF3), calcium fluoride (CaF2), graphite, heavy water (D2O) or the like is generally used. By injecting the neutrons moderated nearly to 1 MeV into these moderators, they are moderated to the epithermal neutron region of the energy suitable for BNCT (4 keV-40 keV).
In the case of the above Non-Patent Document 1 and Non-Patent Document 2, as moderators, Pb, Fe, polyethylene, Al, CaF2 and polyethylene containing LiF are used. The polyethylene and polyethylene containing LiF among them are used as moderators for safety (mainly for shielding) which cover the outside portion of the apparatus in order to prevent leakage of high-energy neutrons out of the radiation field.
It can be said that it is appropriate to moderate the high-energy part of neutrons to some extent using Pb and Fe among these moderators (the first half of the stage of moderation), but it cannot be said that the second half of the stage of moderation using Al and CaF2 after the moderation to some extent is appropriate. That is because the moderators used in the second half of the stage thereof have insufficient shielding performance to fast neutrons, and a high ratio of fast neutrons having a possibility of bad influences on healthy tissues of a patient is left in the moderated neutron type.
By reason of CaF2 having insufficient shielding performance to the high-energy part of neutrons as a moderator used in the second half of the stage thereof, part of them passes without being shielded. The polyethylene containing LiF used with CaF2 in the second half of the stage thereof covers over the entire surface except an outlet of neutrons on the treatment room side. It is arranged so as to prevent whole-body exposure of a patient to the fast neutrons, without having a function as a moderator on the outlet of neutrons. For information, the polyethylene among the moderators in the first half of the stage thereof covers over the entire surface of the periphery of the apparatus except the treatment room side, like the polyethylene containing LiF in the second half of the stage thereof, and it is arranged so as to prevent the fast neutrons from leaking to the surroundings of the apparatus.
Therefore, instead of CaF2 as a shielding member to fast neutrons in the second half of the stage thereof, the development of a moderator which can shield and moderate high-energy-level neutrons while suppressing the attenuation of intermediate-energy-level neutrons required for treatment has been desired.
From various kinds of researches/studies, the present inventors found a MgF2 sintered body or MgF2 system substances, for example, a MgF2—CaF2 binary system sintered body as a moderator which makes it possible to obtain neutrons (neutrons of the energy of 4 keV-40 keV) mainly comprising epithermal neutrons in anticipation of the highest remedial value, from the above neutrons moderated to some extent (the energy thereof is approximately up to 1 MeV). As the MgF2 system substances, a MgF2—LiF binary system sintered body, a MgF2—CaF2—LiF ternary system sintered body other than the MgF2—CaF2 binary system sintered body can be exemplified.
For information, as of now, there has been no report that magnesium fluoride (MgF2) was used as a moderator to neutrons, not to mention that there has been no report that a MgF2 sintered body or a MgF2—CaF2 binary system sintered body was used as such neutron moderator.
The present inventors have filed an application of an invention relating to a sintered body of MgF2 simple (a technical term related to raw material technology, a synonym for “single”) prior to this invention (Japanese Patent Application No. 2013-142704, hereinafter, referred to as Prior Application I). Furthermore, the present inventors have filed an application of an invention relating to a MgF2—CaF2 binary system sintered body as a neutron moderator (Japanese Patent Application No. 2014-193899, hereinafter, referred to as Prior Application II). The Prior Application I and Prior Application II are described later in detail.
The present invention was achieved in order to further improve the characteristics of the MgF2 sintered body and the MgF2—CaF2 binary system sintered body according to the two prior applications.
A single crystal body of MgF2 has high transparency, and high light transmittance within a wide range of wavelengths of 0.2 μm-7 μm, and it has a wide band gap and high laser resistance. Therefore, it has been mainly used as a window material for eximer laser. Or when a MgF2 single crystal body is deposited on the surface of a lens, it shows effects of protection of the inner parts thereof or prevention of irregular reflection. In either case, it is used for optical use.
On the other hand, since the MgF2 sintered body has low transparency because of its polycrystalline structure, it is never used for optical use. Since the MgF2 sintered body has high resistance to fluorine gas and inert gas plasma, a few applications concerning an application thereof to a plasma-resistant member in the semiconductor producing process have been filed. However, there is no publication or report that it was actually used in the semiconductor producing process.
That is because the Bridgman-Stockbarger method (crucible descending method) or the Czochralski method (single crystal pulling method) is adopted in order to manufacture a MgF2 single crystal body, as described above, resulting in the MgF2 single crystal body's strong image of extremely high price, and because a MgF2 sintered body produced by a general sintering method has a low density due to the foaming property of the MgF2 raw material, leading to a tendency to have low mechanical strength, as described in the below-mentioned Patent Document 1.
As for a MgF2 sintered body, according to the Japanese Patent Application Laid-Open Publication No. 2000-302553 (the below-mentioned Patent Document 1), the greatest defect of ceramic sintered bodies of fluoride such as MgF2, CaF2, YF3 and LiF is low mechanical strength. And in order to solve this problem, the invention was achieved, wherein sintered bodies compounded by mixing these fluorides with alumina (Al2O3) at a predetermined ratio can keep excellent corrosion resistance of the fluorides as well as obtain high mechanical strength.
However, as for the corrosion resistance and mechanical strength of the sintered bodies produced by this method, in any combination, the sintered bodies are simply allowed to have just an intermediate characteristic between the characteristic of any of the fluorides and that of alumina. No sintered body having a characteristic exceeding one's characteristic superior to the other's has been obtained by compounding. In addition, their use is limited to high corrosion resistance uses, different from the uses of the present invention.
Another sintered body mainly comprising MgF2 is described in Japanese Patent Application Laid-Open Publication No. 2000-86344 (the below-mentioned Patent Document 2), but its use is also limited to a plasma-resistant member. In the Patent Document 2, a sintered body comprises a fluoride of at least one kind of alkaline earth metals selected from the group of Mg, Ca, Sr and Ba, in which the total amount of metallic elements other than the alkaline earth metals is 100 ppm or less on a metal basis, the mean particle diameter of crystal grains of the fluoride is 30 μm or less, and the relative density is 95% or more.
However, the materials in the list (Table 1) of Examples of the Patent Document 2 were obtained by firing a fluoride of each single kind of the above four alkaline earth metals (i.e. MgF2, CaF2, SrF2 and BaF2), and no fired mixture of those fluorides is described.
Still another example of an application of a sintered body mainly comprising MgF2 to a plasma-resistant member is the Japanese Patent Application Laid-Open Publication No. 2012-206913 (the below-mentioned Patent Document 3). The Patent Document 3 discloses an invention wherein, since a sintered body of MgF2 simple has a defect of low mechanical strength, by mixing at least one kind of non-alkaline metallic dispersed particles having a lower mean linear thermal expansion coefficient than MgF2 such as Al2O3, AlN, SiC or MgO, the defect of low mechanical strength thereof can be compensated for. When a sintered body of such mixture is used as the above moderator to neutrons, the moderation performance thereof is greatly different from that of MgF2 simple because of the influence of the non-alkaline metal mixed into MgF2. Therefore, it is predicted that it is difficult to apply a sintered body of this kind of mixture to a use as a moderator.
In addition, an example of an application of a sintered body mainly comprising CaF2 to a plasma-resistant member is the Japanese Patent Application Laid-Open Publication No. 2004-83362 (the below-mentioned Patent Document 4). The Patent Document 4 describes a method wherein using hydrofluoric acid, impurities other than Mg are removed from a low-purity raw material containing Mg, so as to precipitate high-purity CaF2, and a fluoride sintered body whose starting raw material is the high-purity CaF2 containing Mg of 50 ppm or more and 5% by weight or less is produced. A problem here is a state of Mg contained in the starting raw material, though the state is not described at all. And there is no description concerning the technique by which the degree of purity of the low-purity raw material is raised using hydrofluoric acid.
Then, when presuming the process of raising the degree of purity of a low-purity raw material as a person skilled in the art, generally speaking, in the case of raising the degree of purity of a low-purity raw material using hydrofluoric acid, a method is often adopted, wherein impurities in the raw material are first dissolved into a hydrofluoric acid solution as many as possible, and if a component (Ca, here) desired to be a main raw material dissolved with impurities in this dissolution process, the component is precipitated and separated by use of the difference in solubility among the dissolved components. When further reviewing the invention, it is presumed that Mg exhibited different dissolution behavior from other impurities. In the specification, it is referred to as only “Mg”, and according to the descriptions of Examples in Table 1, as for high concentrations of impurity components other than Mg (such as Fe, Al, Na and Y), all of their concentrations were decreased by purity raising treatment, but only the concentration of Mg did not change, being 2000 ppm before the treatment and being also 2000 ppm after the treatment, leading to the above presumption. Hence, there is a high possibility that Mg might be in a state which is hard to dissolve in hydrofluoric acid, that is, a metal state. If CaF2 containing metal Mg is a starting raw material, the sintering process thereof is very different from the case like the present invention wherein a mixture of CaF2 and MgF2 is a starting raw material, and the characteristics of the sintered bodies are also very different from each other.
On the other hand, inventions relating to a neutron moderator were disclosed lately. One of them is the Japanese Patent No. 5112105 (Patent Document 5). The Patent Document 5 discloses ‘a moderator which moderates neutrons, comprising a first moderating layer obtained by melting a raw material containing calcium fluoride (CaF2), and a second moderating layer comprising metal aluminum (Al) or aluminum fluoride (AlF3), the first moderating layer and the second moderating layer being adjacent to each other’.
In the Patent Document 5, the first moderating layer obtained by melting the raw material containing CaF2 is disclosed, but raw material conditions such as the purity, components, particle size and treatment method thereof, and melting conditions such as heating temperatures, holding times thereof and the type of heating furnace are not mentioned at all, very insincerely described as a patent specification. In the Patent Document 5, there is no description suggesting that something related to MgF2 should be used as a neutron moderator.
Another invention is disclosed in the document by KONONOV, O. E. et al. (Non-Patent Document 4). This Non-Patent Document 4 says that MgF2 has good moderation performance as a neutron moderator. However, there is no description about the MgF2 used therein except for ‘its density of 3.14 g/cm3’. Even how to produce it, conditions for producing it, and whether the mineral texture thereof is a single crystal or polycrystalline or amorphous (vitreous), or a mixture of them, are not mentioned at all. Furthermore, whether it was produced by the writers thereof or by outsiders is not mentioned.
In cases where a polycrystalline article or an amorphous article which is not on the market is used, generally the descriptions such as its origin and the characteristics/physical properties such as its quality other than the density should be included. Therefore, in this case, it is presumed that a ‘single crystal’ article being on the market was used.
In case where the writers made a polycrystalline article of MgF2 on an experimental basis by a sintering method, as described below, it should be concluded that an article having an extremely high density (3.14 g/cm3: relative density of 99.7%) could be made although MgF2 has a high foaming property, and that an article of extremely excellent quality that had never been seen in the past could be made. In such case, naturally the description of such excellent quality should be included in the document. Hence, it can be concluded that as the MgF2 article, a ‘single crystal’ article available in the market or that one can produce was used.
In the preceding documents, as described above, there is no description suggesting the use of a sintered body of MgF2 as a moderator to neutrons, one kind of radioactive rays. In such situation, the present inventors found that it was possible to use a MgF2 sintered body with a modification made thereto as a moderator to neutrons, one kind of radioactive rays, and achieved the invention of the Prior Application I.
In the invention of the Prior Application I, a high-purity MgF2 raw material was pulverized and two-stage compressing and molding was conducted thereon. That is, after molding by a uniaxial press molding method, this press molded body was further molded by a cold isostatic pressing (CIP) method so as to form a CIP molded body. Then, by firing the same in three steps with different heating conditions using an atmospheric furnace in which the atmosphere can be adjusted, a sintered body having a compact structure was produced while suppressing foaming of MgF2 as much as possible. However, since MgF2 very easily foams, it was difficult to actually suppress foaming thereof. As a result, the range of the relative densities (i.e. 100×[bulk density of a sintered body]/[true density](%)) of the sintered bodies produced by this method was 92%-96%, and the mean value thereof was of the order of 94%-95%.
A characteristic desired for a sintered body for a neutron moderator is ‘the mean value of relative densities of 95% or more at least, desirably 96% or more should be stably secured’.
As described above, the fundamental performance required for a moderator used in the BNCT method is ‘to prevent high-energy neutrons such as fast neutrons from leaking, and to sufficiently secure epithermal neutrons necessary for therapy’, and a sintered body satisfying the above characteristic may be a sintered body having the above fundamental performance.
It could not be said that the relative density of the sintered body according to the Prior Application I should lead to the performance thereof sufficient as a moderator used in the BNCT method. Therefore, the present inventors worked toward further development and found out that when using a MgF2—CaF2 binary system sintered body for radiation use, the relative density thereof could be easily improved, compared with a sintered body of MgF2 simple, resulting in an anticipation to improve the performance as a moderator, which led to the invention of the Prior Application II.
In the Prior Application II, after mixing/pulverizing a high-purity MgF2 raw material and a CaF2 raw material, two-stage compressing and molding was conducted thereon. That is, after molding the same by a uniaxial press molding method, this press molded body was further molded by a cold isostatic pressing (CIP) method so as to form a CIP molded body. Then, it was fired in three steps with different heating conditions using an atmospheric furnace in which the atmosphere can be adjusted so as to produce a sintered body having a compact structure while suppressing foaming of MgF2 as much as possible.
However, since a MgF2—CaF2 binary system molded body also easily foams, it was difficult to actually suppress foaming thereof. Consequently, the range of the relative densities of the sintered bodies produced by this method was 94%-97%, and the mean value thereof was of the order of 95%-96%.
It was a level in which the request of ‘the mean value of relative densities of 95% or more at least, as a moderator’ was just cleared, and it was difficult to satisfy the request ‘desirably the mean value of relative densities of 96% or more should be stably secured’.
This MgF2—CaF2 binary system sintered body had a relative density about 1% higher than the MgF2 sintered body in the Prior Application I. However, it was admitted that open pores remained in the periphery portion of the sintered body, judging from a phenomenon in which pure water penetrated the sintered body when it was soaked in pure water for measuring the bulk density thereof. Thus, even in the invention of the Prior Application II achieved in order to improve the invention of the Prior Application I, it was predicted that open pores still remained in the sintered body, and it was proved that there should be yet room for improvement.
As described above, since MgF2 very easily foam, it is not easy to actually suppress foaming thereof. Consequently, it is desired that even the sintered bodies produced by the methods according to the Prior Application I and Prior Application II should have further improved moderation performance to radiation, especially to neutrons, and a higher relative density (i.e. 100×[bulk density of the sintered body]/[true density](%)).